In everyday life, environmental background sound is often present. For instance, we may try to follow a conversation while other people talk in the background or we may try to hear out an announcement in a noisy airport. Our ability to ignore unwanted sound is crucial to helping us navigate these common social settings, yet even mild hearing loss can make it very difficult to ignore background sound. The inability to understand speech in these noisy situations, called masking, can lead to social isolation and depression. Thus, an important unresolved question in auditory neuroscience is to determine how the neural mechanisms operate by which we hear out target sound from an acoustic mixture. Two principal types of masking interfere with optimal function of hearing aids. The first type, called energetic masking is well characterized through psychophysics, physiology and modeling. The second type, called informational masking, is much less understood. We lack comprehensive knowledge linking perception and neuronal activity associated with informational masking. This proposal aims to identify behavioral and neuronal mechanisms of informational masking. We propose to examine cortical mechanisms of informational masking in humans and in an animal model, the Mongolian gerbil (Meriones unguiculatus). By combining empirical approaches in psychoacoustics and in-vivo physiology recordings from awake animals, the key goal of this proposal is to improve our understanding on how the auditory cortex processes a challenging task such as informational masking. We will examine the neuronal mechanisms of informational masking, based on auditory cortex responses, and determine how individuals overcome informational masking with vs. without hearing loss. First, we will test normal-hearing human listeners as well as gerbils under conditions of informational masking and simultaneously record from auditory cortex. In humans, we will record the hemodynamic response of blood oxygenation, using functional near infrared spectroscopy, a quick and robust assessment technique with clinical relevance. In gerbils, we will measure neuronal activity in core auditory cortex from trained animals. We will use this data to develop an objective metric of an individual?s vulnerability to informational masking. Second, we will examine the neuronal mechanisms of informational masking by introducing rapid unpredictable changes in background sound and assessing if high vulnerability to informational masking is due to predominant reliance on suppressing background activity (as opposed to enhanced responses to the target) in humans and gerbils. Third, using our animal model, we will test how hearing loss affects susceptibility to informational masking. Collectively, this proposal will functionally define informational masking at both perceptual and cortical processing levels. The results are expected to significantly advance our understanding of the origins and scope of central auditory processing deficits in common everyday situations with background sound.